Radio frequency amplifier



Sept. 4,' 1928.

F. K. VREELAND 'RADIO FREQUENCY AMPLIFIER- 3 Sheets-Sheet Filed Alig. 6,1923 v INVENTOR Sept. 4, 1928. 1,682,874

' F. K. VREELAND RADIO FREQUENCY AMPLIFIER Filed Aug. 1923 3Sheets-Sheet 2 mu) IIIIII/m VEN TOR Sept. 4, 1928. 1,682,874

F. K. VREELAND RADIO FREQUENCY AMPLIFIER Filed Aug. 1923 3SheetsSheet 33 i l l l l l l l PFI 0 INVENTOR I WWW Patented Sept. 4, 1928. V

UNITED' STATES 1,682,874 PATENT OFFICE.

FREDERICK K.:-VREELAND, OF MOIIIDCLAIR,'NEW JERSEY, ASSIGNOR TO VREELANDCORPORATION, OF HOBOKEN, NEW JERSEY, A CORPORATION OF NEW JERSEY.

RADIO FREQUENCY AMPLIFIER.

Application filed August 6,- 1923. Serial No. 655,794.

The invention herein described relates to amplifiers for high or radiofrequency currents and is particularly concerned with devices forcoupling'amplifier tubes in cascade.

It has for one of its objects the elimination or minimizing of theeffects of detrimental capacity and stray fields, and the production ofan amplifier of greatly improved efliciency.

In my U. S. Patent No. 1,666,518, issued upon a division of the presentapplication, are described and claimed certain forms of apparatusadapted for the practice of the method to which the present claims ofthis application are particularly directed, to wit,

the method of accomplishing band amplification and specifically to thecontrol of the band characteristic, whereby the amplification is madeeffective or substantially uniform for all frequencies within a givenband or range, while frequencies outside this band are almost completelyexcluded. In accordance with one phase of the invention reactances, orspecifieally capacities are added in successive stages of amplificationand such reactances or capacities -are adjusted or changed for thepurpose of altering the frequency characteristic of the amplifier andparticularly for shifting the position of the band over which itoperates in the frequency scale. In

accordance with another phase of the invention the polarity of theamplified output is reversed for the purpose of establishing a 5 phaserelation of the currents in the several stages of amplification whichminimizes the tendency to regeneration and oscillation.

' In the drawing Fig. 1 is a schematic diagram showing a simple form ofapparatus to embodying the invention. Figs. 1, 1 and 1 show alternativearrangements of certain features of Figs. 1, 2 and 6.

Fig. 2 is a modified form in which the astatic feature is included.

L5 Fig. 3 is a more detailed diagram showing thephysical construction ofone type of coupling transformer.

. Figs. land 4 show the structural features of another type oftransformer embodying i the invention.

' Fig. 5 shows the amplification characteristics secured by applicationof the invention.

Fig. 6 is a schematic diagram of a multiple band amplifier.

.in stray capacities, so that only a fraction,

including. features One of the most serious limitations in the design ofradio frequency transformers, as usually practiced, is the fact that thesecondary clectromotive force is largely dissipated and sometimes asmall fraction of it is useful in producing amplification. Usually it isconsidered impracticable to construct such a transformer having astep-up ratio, be-

cause the electromotive force gained by the step-up ratio is fritteredaway in these stray capacities, including the capacity of thetransformer itself.

By means of the present invention I not only avoid the detrimentaleffect of capacity in the windings, but I utilize the capacity toproduce an improved result. By means of this invention I am enabled toconstruct a transformer of high efficiency with a stepup ratio.

Another difficulty in radio frequency amplifiers of the usual type isinter-action of the various stages, when amplifiers are used in cascade,which reduces the effective amplification and tends to instability oroscillation ofthe system. An important feature of the present inventionis an arrangement of windings whereby the external field of thetransformer is reduced to a minimum so that it becomes practicallyastatic and inductive disturbances are eliminated.

A particular feature of the present invention resides in performing theamplification in successive stages having differentamplication-frequency characteristics, which are preferably spaced inthe frequency scale with an overlap sorclated to the width of thecharacteristics that the combined amplifying effect of the system issubstantially uniform for a given band of frequencies. The specificfeature relates to the shifting of this band in the frequency scalewhile preserving its band character.

In the drawing Fig. 1 shows schematically a simple embodiment of theinvention, as 100 applied to a radio frequency amplifier of three tubes.In the arrangement shown, two of the tubes A,, A, are'used for radiofrequency amplification and the third D as a detector. When in thefollowing I refer to amplifier tubes generically it will be understoodthat one of these may be used for detection asshown. TT are transformerscoupling the tubes in cascade, each ineluding a primary coil 12 and asecondary indicating that the'polarities of coil 8. In the firsttransformer the outer terminal of the primary coil p is connected to theoutput terminal or anode of the first amplifier tube A and the outerterminal. of the secondary s is connected to the input terminal or gridof the second amplifier tube A,. The second transformer is similarlyconnected to the second amplifier tube A, and the detector tube D. Theprimary and secondary coils are wound or connected in opposite senses sothat their outer terminals aand b respectively are at oppositepotentials, instead of being at like potentials as is usual in theconstruction of such transformers. This feature is indicated in the diagram by the sign F on the primary terminal a, and i on the secondaryterminal 5, these termi-v nals change in opposite senses. The resultingpotential difference between these terminals, which would ordinarily bedetrimental, is. utilized to assist instead of opposing the Thus thetransformer magnetic coupling. may be so constructed that the primaryand secondary windings constitute in effect a condenser, whose capacityaugments or assists the magnetic coupling effect of the windings, aswill be explained. This inherent capacity of the windings is indicatedby the dotted capacity couplings C in the drawings, dotted lines beingemployed to indicate that the capacities, although physically andnecessarily existant as capacities, do not necessarily require separatestructural elements, as usually employed, but may be in herent in thetransformer structure. If desiredan external condenser may be connected'across the terminals a b to augment the inherent capacity of thetransformer windings as shown at 8 in Fig. 1.

This capacity, whether inherent in the windings or external, assists themagnetic coupling of the transformer. It may be so chosen that-itsefi'ective reactance at the preferred operating frequency isapproximately equal to the combined reactances of the coils p8, so thatthe transformer becomes in effect an oscillating circuit ps0, tunedapproximately tothe operating frequency, due allowance being made forthe capacities of the associated tubes when these are material withrespect to the capacity of the transformer. The result of thisarrangement is that the full potential difference between the terminalsa. b is e fective in coupling theoutput of one tube with theinput of thenext.

The windings ye may be similar, in which case the usefulsecondarypotential will be much larger than thatof an equal ratiotransformer not embodying this invention, but preferably the secondaryis made with more turns than the primary so that a stepup ratio issecured.

The significance of this arrangement will be seen more clearly byreference to Fig. 1*, which is .a simplified schematic diagram showingthe relations of one pair of tubes to the coupling system. Theinductance of the coupling system comprises the inductance of theprimarycoil p and that of the secondary coil 8, together'with their mutualinductance, and the capacity C is-the inherent or the total capacity ofthe-system. The

.oscillating system 108C is tapped at three points, a, b, and 0, point 0being the negative bus or neutral point. The input electromotive forceis applied between a and 0, and the output elcctromotive force is takenoff between points I) and 0. The-ratio between the input and outputelectromotive forces is determined by the inductive drops across coils pand s, which may be given any desired value within wide practical limitsby suitably proportioning the coils.

To secure the best results it is usually desirable ,to shunt theexternal connections of the transformers including batteries,resistances, etc., by bridging condensers such as c whose capacityreactance in the working range of frequency is small. These serve toby-pass the high frequency oscillations across the inner terminals ofthe transformers, avoiding external loss by completing the closedcircuit including the windings p8 and the mutual capacity of the coilsby the shortest possible route. Other bridging condensers such as c maybe used when desired to bypass the output current of vone tube and theinput current of the next to the filament or ground bus.

Fig. 2 shows an lmproved arrangement which includes the featurescontained in F1g.,1 and has in addition an astatic feature whereby theexternal field of the transtively close coupling is obtained betweenprimary and secondary through their lunbalanced internal fields,notwithstanding the opposing relations of the two halves of thesewindings which cause their external fields to balance and neutralizeeach other. The sections 12 8, are wound or connected in 01 positesenses, as explained in connection with Fig 1, and the same is true ofthe sections 7 8 so that the external terminals a b will have oppositepolarities as in the arrangementof Fig. 1. The physical dimensionsandrelative positions of the coils may be so chosen, as in the case of Fig.1, that the effective capacity between the terminals a 7) has areactance at the preferred operating frequency approximately equal tothe inductance reactance, so that at this preferred frequency themaximum effective otential difference is obtained between terminals (1b.

In this arrangement it will be noted that while the mutual induction ofthe two windings is relatively large, their external mag-' mutualinteraction is avoided.

There are various forms of construction that can be used in carrylng outv the inven tion.

used. Here the primary coil is made in two halves, 11,10 wound inopposite directions as shown, and the secondary coil is also woundin-two opposing halves 8,8, as lndicated. The

' primary and secondary coils are shown sepvalue. v

tion of windmg of the 0011s which gives the arated for clearness ofillustration of the windings. In operation the primary coil is placedinside the secondary, or vice versa, and the dimensions of the windingsand diameters of the tubes are so chosen that the capacity betweenwindings has the requisite The drawing shows also the direcby windingfirst the desired number of turns in one direction for the inner half ofthe coil, as 10,, then winding silk thread toform a spacer G and finallywinding the second section of the coil 39, in the opposite direction.The same construction is used in the second? ary coil 8, 8 though inthis case preferably the number of turns in each section is greater.

The dimensions of the coils and the dis tance between them are so chosenthat the capacity has the desired value in relation to the inductance ofthe windings.

Either of the constructions shown in Figs. 3, 4 and 4? may be used forthe transformers TT in Figs. 2 and 6, or any other suitableconstruction, preferably embodying the fea- One of these is shown inFig. 3, in which cylindrical or solenoidal windings are tures hereindescribed, may be used, although the major features of the inventionparticularly claimed herein may be carried out with any suitablecoupling means. Where coupling transformers are used as shown, a greatvariety of constructions may be employed, permitting the choice of anydesired amplification frequency characteristic for the individualamplifier units and their coupling means, and so providing considerablelatitude in the combined or overall band characteristic. Thus by windingthe transformer coils with low resistance the apparatus may be madehighly selective, so that the transformer circuit with its inherent andassociated capacities is resonant to a definite frequency and theamplification is high for this frequency but lower for any otherfrequency. The individual characteristic curve of such an amplifier unitis shown at a. Fig. 5 in which the ordinates represent the ratio ofamplification and the abscissas represent the frequency. On the otherhand, where an amplifier is desired which works effectively over a widerange of frequencies I prefer to wind the transformer coils or particularly the secondary coil of relatively high resistance; for example bywinding them of fine copper or larger German silver wire. Thisresistance has the effect of lowering the peak of the amplificationcurvewithout greatly lowering its amplitude at frequencies dif: ferentfrom the; peak frequency, so that a broad characteristic is securedwithout corresponding loss in efficiency, as shown for example in b Fig.5.

The degree of coupling between the coils is also an important factor indetermining the characteristics of the several amplifier units. Where abroad characteristic is desired I prefer to make the coupling as closeas practicable. WVhere a sharply selective characteristic is desired alooser couplingis permissible. By suitably proportioning the couplingand resistance, as well as the inductance and mutual capacity of thewind ings, the characteristics of the amplifier may be determined atwill.

-A substantially flat combined or overall characteristic may be obtainedover a desired band of frequencies by the use of amplifier units'incascade having individual charac-' teristics which are different. Forexample, Fig. 5, at 0 shows a typical characteristic of a three stageband amplifier. The curves in broken lines 1, 2, 3 show thecharacteristics of the three individual units including tubes andtransformers Orr other coupling means that are connected in cascade,while the full line curve 4 represents the resultant or overall bandcharacteristic of the combined system.

' The different characteristics of the individual units of a bandamplifier may be determined by'the design of the windings,

" variable.

orcharacteristics like those shown in 1, 2, 3 at c in Fig. 5 may beobtained from transformers of identical windings by suitably choosingthe distance between the coils and the resulting mutual capacity or byother suitable means.

A convenient means of adjusting the characteristic of a transformer atwill is by the use of condensers. Three ways of doing this are shown inFig. 1. The first is by shunting an external condenser 8 across theouter terminals of the primary and secondary coils p, p, s 8,. Thisexternal capacity is in effect added to the inherent mutual capacity ofthe coils, and so lowers the frequency characteristic. The secondarrangement consists in connecting a condenser 9 in parallel with theprimary coil 12, 1),. This has a result equivalent to increasing theeffective inductance of the primary coil and consequently it lowers thefrequency characteristic. Ansimilar result is obtained by connecting acondenser 10 across the secondary coil s 8' but the effect of suchcapacity is proportionately greater than when the same capacity is addedat 9 if, as l prefer, the transformer coils p, 12 s 8 have astepupratio.

The condensers or capacities 8, 9, 10 may be used singly or in anydesired combination, and any or all of them may be made An effectivearrangement for tuning the coupling transformers is secured by designingthe transformer to have a sharply selective characteristic and insertinga variable condenser at 9.

A particularly useful modification of the band amplifier is shown inFig. 6. Here are three transformers with their corresponding tubesarranged in cascade. These transformers are preferably .designed to haveoverlapping characteristics as shown at c in Fig. 5, giving a combinedband characteristic which is flat. It will be observed from this figurethat when the spacing of the individual characteristics is suitablyrelated to the width or form of the characteristics, the resultantcharacteristic of over-all amplification is a substantially flat bandwith a sharp cut-off at each side. The desired spacing may be secured byany of the means herein described for determining thefrequency'characteristic of the amplifier, or by any other suitablemeans. This characteristic is shifted in the frequency coordinate byadding impedances to the transformer systems, .prefe'rably by the use ofsmall condensers 9 9 of suitably proportioned capacity so arranged thatthey may be shunted successively across 'the transformers. Thesecondensers are cut in and out by switches S S, the switches S S S beingarranged to be operated simultaneously, for example by mounting them ona common rock shaft r 11 operated by a handle or lever 12. Similarly theswitches S" S" S" operate simul-- taneously. When the switches Sareclosed by rocking the shaft, cutting in-the condensers 9, thecharacteristics of all the transformers are changed in frequency so thatthe combined characteristic is a flat topped band displaced in thefrequency coordinate as shown at 6 in part cl Fig.5. Additionalcondensers 9" may be switched on by a further movement of the rock-shaftll, closing the switches S, and in this way the operating range of theamplifier may be broadened as far as desired. Preferably the capacitiesof the condensers 9 9 are so chosen that the bands 5, 6, 7 have a slightoverlap, as shown.

This arrangement tages, particularly when used for selectivelyamplifying high frequency signal waves and eliminating interference. Anamplifier is has numerous advan-' secured which gives practicallyuniform amplification over a wide ran e of frequencies, and thesub-division of this range into a plurality of narrower bands is usefulin eliminating interference from signals of .undesired frequencies,since the amplifier is highly effective for frequencies within its handbut excludes almost completely frequencies outside of this band. By thismeans extremely feeble signals may be received without interference fromthe most powerful nearby stations.

While'I prefer to shunt the band-shifting condensers 9' 9 across theprimary coils 2 p, as shown, they ma be placed, if desired, in any ofthe positions 8, 9, 10, shown in- Fig. l. This feature is notspecifically claimed in this application, but is specifically claimed ina separate application.

The flatness of the band of effective amplification and the sharpness ofthe cut-off for frequencies outside this band, which characterize thepreferred form of my invention,

are secured by so relating the spacing of the characteristics oftheseveral amplifier stages to the width and form of the characteristics asto secure the particular overlap that'is required for the characteristicemployed. This relation is illustrated in F ig. 5, a, which shows curvesfor 'a three-stage am- Considering the relation of these curves atdifferent frequencies, it is noted that for a frequency less than thepeak frequency Y of the characteristic 1, characteristic 3 showssubstantially no amplification and the over-all characteristic is small.-As the frequency'is increased, all the component characteristics areincreasing simultaneously, and the amplification rises very 'sharply'upto a frequency corresponding to the peak of curve 1. At this frequencycurve 1 begins to fall while the others rise more slowly, and, by virtueof the particular s acing employed, the rise of one curve su stantiallyoffsets the fall of another, so that the am plification is substantiallyuniform, up to a point corresponding approximately .to the peak of curve3, beyond which all of the slope of the component characteristicsso thatthe width of the curve at its base is not materially greater than thatof the component characteristics, notwithstanding the breadth andflatness of the band at its top.

This is of great importance since it permits a high degree ofselectivity due to the sharp cut-off, notwithstanding the widthof theband of effective amplification.

The flatness of the band depends upon the spacing. If the spacing iscloser than the optimum, the over-all characteristic will be peaked, ifthe spacing is broader than the optimum, the over-all characteristicwill I have a central valley. Usually the optimum spacing is secured'fora 3-stage amplifier when the high and low frequency'characteristics (3andl, Fig. 5, 0 intersect and overlap at apoint corresponding to abouthalf the maximum amplification. For other ampllfiers the optimum spacingis readily determined by computation or' graphic methods.

I claim as my invention:

1. The method of amplifying high fre-- quency currents which consists inamplifying the current energy, inductively reversing the polarity of theamplified output, shunting the reversinginductance by a capacity whosereactance balances the reactance of the reversing inductance .at a givenfrequency, and adding capacity to the system whereby its reactances arebalanced at a different frequency.

2. The method of amplifying high fre quency eurrents'which consists inamplify ing the current energy, inductively reversing the polarity ofthe amplified output,

shunting the reversing inductance ,by a capacity whose reactancebalances the reactance of the reversing inductance at a given frequency,and adding successive increments ofv capacity to the system whereby itsreacting increments of capacity to the several ances are balanced atsuccessively different frequencies.

3. The method of amplifying hi'gh frequency currents which consists inperforming successive'stages of amplification having differentamplification-frequency characteristics and in combination producingeffective amplification over a given band of frequencies, shifting thefrequency of this band wliile preserving its hand character by acid ingincrements of reactance to the several stages of amplification,receiving currents of a-given frequency within the desired band, andexcludingfrequencies outside this band; 4. The method of amplifying highfrequency currents which consists Sin-performing successive stages ofamplification having different amplificationfrequenc characteristics andin combination p'ro ucing effective amplification over a givemband offrequencies, shifting the-frequency of this band while preserving itshand character by add stages of amplification, receiving currents of andexcluding frequencies outside-this band.

5. The method of selectively amplifying high fre uency signal waves andeliminating inter erence which consists in performing successivestagesof amplification having different frequency characteristics, and,spacingthe frequency characteristics of the successive stages inthefrequency scale with an overlap so related to the width of thecharacteristics as to produce in combination a band of substantiallyuniform amplificatlon with a sharp cut-off at frequencies outside theband.

6. The method of selectively amplifying high frequency currents whichconsists in performlng successive stagesof amplification having dfferent selective character- 1t1'cs, spaclng the characterlstlcs 1n thefrequency scale with an overlap so related to the. width of thecharacteristics asto pro duceeffectiveamplification over a band offrequencies with a sharp cut-off at fre-' quencies outside this band,and shifting the characteristics in the frequency scale whilemaintaining the spacing relation, thereby shifting the band of effectiveamplification in the frequency scale while preserving its handcharacter.

This specification signed this 31st day of July, A. D. 1923. t FREDERICKK. VREELAND.

80 a given frequency within the desired band, 7

